There are known and used for the above-referred to purpose canned zero-leakage pumps. Such zero-leakage pumps have certain advantages over other types of pumps such as the absence of leakage and less maintenance. However, in systems such as installations in which heat transfer is required and crucial, certain important safety requirements may need to be provided. In the event that the pump motor stops, for instance due to power failure, dangerous temperature may occur in the system to be cooled by circulation of the coolant.
It is known and common practice to mount a flywheel keyed to the drive shaft of the pump motor. The purpose of such flywheel is to utilize the inertia of the flywheel to maintain the circulation of the coolant in case of power failure or breakdown of the motor for a period of time sufficient to stop the system to be cooled. Such arrangement presents no difficulties when a dry electric motor with a seal about its shaft is used. However, with a zero-leakage pump driven by a submersible motor in which both the pump and the drive motor are housed in the same pressure enclosure filled with the coolant to be circulated, it has been found that when a flywheel is mounted on the shaft of such motor the flywheel entails large consumption of the motor power due to the hydraulic drag friction as caused by rotation of the flywheel in the coolant such as water. This lowers the combined efficiency of the pump, motor and flywheel very considerably. As a result, the use of zero-leakage pumps in heat-transfer requiring systems in which continuous circulation of a coolant is crucial to avoid dangerous heat build-up has often been found to be unacceptable even though the use of such pumps would be highly desirable.